The recovery of 2-pyrrolidone from aqueous solution is inherently difficult because of the hydrophilic character of this lactam, a 5-membered ring of the formula: ##STR1##
In the polymerization of 2-pyrrolidone using an alkaline catalyst, the recovery of unreacted monomer can be accomplished by washing the polymerization reaction mixture with water, or another 2-pyrrolidone miscible solvent, to extract the unreacted 2-pyrrolidone and the alkaline catalyst. The resultant alkaline aqueous 2-pyrrolidone solution may be directly distilled. However, prolonged exposure to alkaline aqueous solutions under distillation conditions can hydrolyze the sensitive 2-pyrrolidone ring thereby depressing the yield of 2-pyrrolidone.
Concentrated acidic aqueous solutions of higher lactams, such as caprolactam, have been found to separate upon neutralization with hydroxides or alkali metal carbonates (see U.S. Pat. Nos. 2,313,026, 2,579,851 and 2,667,483). U.S. patent application Ser. No. 224,284, filed Feb. 7, 1972, teaches that among sodium and potassium salts including the chloride, sulfate, carbonate, nitrate, acetate and thiocyanate, only the carbonate separates aqueous 2-pyrrolidone solutions.